/*
 * Copyright (C) 2001 Momchil Velikov
 * Portions Copyright (C) 2001 Christoph Hellwig
 * Copyright (C) 2005 SGI, Christoph Lameter
 * Copyright (C) 2006 Nick Piggin
 * Copyright (C) 2012 Konstantin Khlebnikov
 * Copyright (C) 2016 Intel, Matthew Wilcox
 * Copyright (C) 2016 Intel, Ross Zwisler
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2, or (at
 * your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

#include <seminix/bitmap.h>
#include <seminix/bug.h>
#include <seminix/cpu.h>
#include <seminix/errno.h>
#include <seminix/idr.h>
#include <seminix/init.h>
#include <seminix/percpu.h>
#include <seminix/radix-tree.h>
#include <seminix/rcupdate.h>
#include <seminix/slab.h>
#include <seminix/string.h>

/*
 * Radix tree node cache.
 */
struct kmem_cache *radix_tree_node_cachep;

/*
 * The radix tree is variable-height, so an insert operation not only has
 * to build the branch to its corresponding item, it also has to build the
 * branch to existing items if the size has to be increased (by
 * radix_tree_extend).
 *
 * The worst case is a zero height tree with just a single item at index 0,
 * and then inserting an item at index ULONG_MAX. This requires 2 new branches
 * of RADIX_TREE_MAX_PATH size to be created, with only the root node shared.
 * Hence:
 */
#define RADIX_TREE_PRELOAD_SIZE (RADIX_TREE_MAX_PATH * 2 - 1)

/*
 * The IDR does not have to be as high as the radix tree since it uses
 * signed integers, not unsigned longs.
 */
#define IDR_INDEX_BITS		(8 /* CHAR_BIT */ * sizeof(int) - 1)
#define IDR_MAX_PATH		(DIV_ROUND_UP(IDR_INDEX_BITS, \
                        RADIX_TREE_MAP_SHIFT))
#define IDR_PRELOAD_SIZE	(IDR_MAX_PATH * 2 - 1)

/*
 * The IDA is even shorter since it uses a bitmap at the last level.
 */
#define IDA_INDEX_BITS		(8 * sizeof(int) - 1 - ilog2(IDA_BITMAP_BITS))
#define IDA_MAX_PATH		(DIV_ROUND_UP(IDA_INDEX_BITS, \
                        RADIX_TREE_MAP_SHIFT))
#define IDA_PRELOAD_SIZE	(IDA_MAX_PATH * 2 - 1)

/*
 * Per-cpu pool of preloaded nodes
 */
struct radix_tree_preload {
    unsigned nr;
    /* nodes->parent points to next preallocated node */
    struct radix_tree_node *nodes;
};
static DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) = { 0, };

static inline struct radix_tree_node *entry_to_node(void *ptr)
{
    return (void *)((unsigned long)ptr & ~RADIX_TREE_INTERNAL_NODE);
}

static inline void *node_to_entry(void *ptr)
{
    return (void *)((unsigned long)ptr | RADIX_TREE_INTERNAL_NODE);
}

#define RADIX_TREE_RETRY	XA_RETRY_ENTRY

static inline unsigned long
get_slot_offset(const struct radix_tree_node *parent, void __rcu **slot)
{
    return parent ? slot - parent->slots : 0;
}

static unsigned int radix_tree_descend(const struct radix_tree_node *parent,
            struct radix_tree_node **nodep, unsigned long index)
{
    unsigned int offset = (index >> parent->shift) & RADIX_TREE_MAP_MASK;
    void __rcu **entry = rcu_dereference_raw(parent->slots[offset]);

    *nodep = (void *)entry;
    return offset;
}

static inline gfp_t root_gfp_mask(const struct radix_tree_root *root)
{
    return root->xa_flags & __GFP_BITS_MASK;
}

static inline void tag_set(struct radix_tree_node *node, unsigned int tag,
        int offset)
{
    __set_bit(offset, node->tags[tag]);
}

static inline void tag_clear(struct radix_tree_node *node, unsigned int tag,
        int offset)
{
    __clear_bit(offset, node->tags[tag]);
}

static inline int tag_get(const struct radix_tree_node *node, unsigned int tag,
        int offset)
{
    return test_bit(offset, node->tags[tag]);
}

static inline void root_tag_set(struct radix_tree_root *root, unsigned tag)
{
    root->xa_flags |= (__force gfp_t)(1 << (tag + ROOT_TAG_SHIFT));
}

static inline void root_tag_clear(struct radix_tree_root *root, unsigned tag)
{
    root->xa_flags &= (__force gfp_t)~(1 << (tag + ROOT_TAG_SHIFT));
}

static inline void root_tag_clear_all(struct radix_tree_root *root)
{
    root->xa_flags &= (__force gfp_t)((1 << ROOT_TAG_SHIFT) - 1);
}

static inline int root_tag_get(const struct radix_tree_root *root, unsigned tag)
{
    return (__force int)root->xa_flags & (1 << (tag + ROOT_TAG_SHIFT));
}

static inline unsigned root_tags_get(const struct radix_tree_root *root)
{
    return (__force unsigned)root->xa_flags >> ROOT_TAG_SHIFT;
}

static inline bool is_idr(const struct radix_tree_root *root)
{
    return !!(root->xa_flags & ROOT_IS_IDR);
}

/*
 * Returns 1 if any slot in the node has this tag set.
 * Otherwise returns 0.
 */
static inline int any_tag_set(const struct radix_tree_node *node,
                            unsigned int tag)
{
    unsigned idx;
    for (idx = 0; idx < RADIX_TREE_TAG_LONGS; idx++) {
        if (node->tags[tag][idx])
            return 1;
    }
    return 0;
}

static inline void all_tag_set(struct radix_tree_node *node, unsigned int tag)
{
    bitmap_fill(node->tags[tag], RADIX_TREE_MAP_SIZE);
}

/**
 * radix_tree_find_next_bit - find the next set bit in a memory region
 *
 * @addr: The address to base the search on
 * @size: The bitmap size in bits
 * @offset: The bitnumber to start searching at
 *
 * Unrollable variant of find_next_bit() for constant size arrays.
 * Tail bits starting from size to roundup(size, BITS_PER_LONG) must be zero.
 * Returns next bit offset, or size if nothing found.
 */
static __always_inline unsigned long
radix_tree_find_next_bit(struct radix_tree_node *node, unsigned int tag,
             unsigned long offset)
{
    const unsigned long *addr = node->tags[tag];

    if (offset < RADIX_TREE_MAP_SIZE) {
        unsigned long tmp;

        addr += offset / BITS_PER_LONG;
        tmp = *addr >> (offset % BITS_PER_LONG);
        if (tmp)
            return __ffs(tmp) + offset;
        offset = (offset + BITS_PER_LONG) & ~(BITS_PER_LONG - 1);
        while (offset < RADIX_TREE_MAP_SIZE) {
            tmp = *++addr;
            if (tmp)
                return __ffs(tmp) + offset;
            offset += BITS_PER_LONG;
        }
    }
    return RADIX_TREE_MAP_SIZE;
}

static unsigned int iter_offset(const struct radix_tree_iter *iter)
{
    return iter->index & RADIX_TREE_MAP_MASK;
}

/*
 * The maximum index which can be stored in a radix tree
 */
static inline unsigned long shift_maxindex(unsigned int shift)
{
    return (RADIX_TREE_MAP_SIZE << shift) - 1;
}

static inline unsigned long node_maxindex(const struct radix_tree_node *node)
{
    return shift_maxindex(node->shift);
}

static unsigned long next_index(unsigned long index,
                const struct radix_tree_node *node,
                unsigned long offset)
{
    return (index & ~node_maxindex(node)) + (offset << node->shift);
}

/*
 * This assumes that the caller has performed appropriate preallocation, and
 * that the caller has pinned this thread of control to the current CPU.
 */
static struct radix_tree_node *
radix_tree_node_alloc(gfp_t gfp_mask, struct radix_tree_node *parent,
            struct radix_tree_root *root,
            unsigned int shift, unsigned int offset,
            unsigned int count, unsigned int nr_values)
{
    struct radix_tree_node *ret = NULL;

    /*
     * Preload code isn't irq safe and it doesn't make sense to use
     * preloading during an interrupt anyway as all the allocations have
     * to be atomic. So just do normal allocation when in interrupt.
     */
    if (!in_interrupt()) {
        struct radix_tree_preload *rtp;

        /*
         * Even if the caller has preloaded, try to allocate from the
         * cache first for the new node to get accounted to the memory
         * cgroup.
         */
        ret = kmem_cache_alloc(radix_tree_node_cachep,
                       gfp_mask | __GFP_NOWARN);
        if (ret)
            goto out;

        /*
         * Provided the caller has preloaded here, we will always
         * succeed in getting a node here (and never reach
         * kmem_cache_alloc)
         */
        rtp = this_cpu_ptr(&radix_tree_preloads);
        if (rtp->nr) {
            ret = rtp->nodes;
            rtp->nodes = ret->parent;
            rtp->nr--;
        }
        goto out;
    }
    ret = kmem_cache_alloc(radix_tree_node_cachep, gfp_mask);
out:
    BUG_ON(radix_tree_is_internal_node(ret));
    if (ret) {
        ret->shift = shift;
        ret->offset = offset;
        ret->count = count;
        ret->nr_values = nr_values;
        ret->parent = parent;
        ret->array = root;
    }
    return ret;
}

void radix_tree_node_rcu_free(struct rcu_head *head)
{
    struct radix_tree_node *node =
            container_of(head, struct radix_tree_node, rcu_head);

    /*
     * Must only free zeroed nodes into the slab.  We can be left with
     * non-NULL entries by radix_tree_free_nodes, so clear the entries
     * and tags here.
     */
    memset(node->slots, 0, sizeof(node->slots));
    memset(node->tags, 0, sizeof(node->tags));
    INIT_LIST_HEAD(&node->private_list);

    kmem_cache_free(radix_tree_node_cachep, node);
}

static inline void
radix_tree_node_free(struct radix_tree_node *node)
{
    call_rcu(&node->rcu_head, radix_tree_node_rcu_free);
}

/*
 * Load up this CPU's radix_tree_node buffer with sufficient objects to
 * ensure that the addition of a single element in the tree cannot fail.  On
 * success, return zero, with preemption disabled.  On error, return -ENOMEM
 * with preemption not disabled.
 *
 * To make use of this facility, the radix tree must be initialised without
 * __GFP_DIRECT_RECLAIM being passed to INIT_RADIX_TREE().
 */
static int __radix_tree_preload(gfp_t gfp_mask, unsigned nr)
{
    struct radix_tree_preload *rtp;
    struct radix_tree_node *node;
    int ret = -ENOMEM;

    preempt_disable();
    rtp = this_cpu_ptr(&radix_tree_preloads);
    while (rtp->nr < nr) {
        preempt_enable();
        node = kmem_cache_alloc(radix_tree_node_cachep, gfp_mask);
        if (node == NULL)
            goto out;
        preempt_disable();
        rtp = this_cpu_ptr(&radix_tree_preloads);
        if (rtp->nr < nr) {
            node->parent = rtp->nodes;
            rtp->nodes = node;
            rtp->nr++;
        } else {
            kmem_cache_free(radix_tree_node_cachep, node);
        }
    }
    ret = 0;
out:
    return ret;
}

/*
 * Load up this CPU's radix_tree_node buffer with sufficient objects to
 * ensure that the addition of a single element in the tree cannot fail.  On
 * success, return zero, with preemption disabled.  On error, return -ENOMEM
 * with preemption not disabled.
 *
 * To make use of this facility, the radix tree must be initialised without
 * __GFP_DIRECT_RECLAIM being passed to INIT_RADIX_TREE().
 */
int radix_tree_preload(gfp_t gfp_mask)
{
    return __radix_tree_preload(gfp_mask, RADIX_TREE_PRELOAD_SIZE);
}

/*
 * The same as above function, except we don't guarantee preloading happens.
 * We do it, if we decide it helps. On success, return zero with preemption
 * disabled. On error, return -ENOMEM with preemption not disabled.
 */
int radix_tree_maybe_preload(gfp_t gfp_mask)
{
    return __radix_tree_preload(gfp_mask, RADIX_TREE_PRELOAD_SIZE);
}

static unsigned radix_tree_load_root(const struct radix_tree_root *root,
        struct radix_tree_node **nodep, unsigned long *maxindex)
{
    struct radix_tree_node *node = rcu_dereference_raw(root->xa_head);

    *nodep = node;

    if (likely(radix_tree_is_internal_node(node))) {
        node = entry_to_node(node);
        *maxindex = node_maxindex(node);
        return node->shift + RADIX_TREE_MAP_SHIFT;
    }

    *maxindex = 0;
    return 0;
}

/*
 *	Extend a radix tree so it can store key @index.
 */
static int radix_tree_extend(struct radix_tree_root *root, gfp_t gfp,
                unsigned long index, unsigned int shift)
{
    void *entry;
    unsigned int maxshift;
    int tag;

    /* Figure out what the shift should be.  */
    maxshift = shift;
    while (index > shift_maxindex(maxshift))
        maxshift += RADIX_TREE_MAP_SHIFT;

    entry = rcu_dereference_raw(root->xa_head);
    if (!entry && (!is_idr(root) || root_tag_get(root, IDR_FREE)))
        goto out;

    do {
        struct radix_tree_node *node = radix_tree_node_alloc(gfp, NULL,
                            root, shift, 0, 1, 0);
        if (!node)
            return -ENOMEM;

        if (is_idr(root)) {
            all_tag_set(node, IDR_FREE);
            if (!root_tag_get(root, IDR_FREE)) {
                tag_clear(node, IDR_FREE, 0);
                root_tag_set(root, IDR_FREE);
            }
        } else {
            /* Propagate the aggregated tag info to the new child */
            for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
                if (root_tag_get(root, tag))
                    tag_set(node, tag, 0);
            }
        }

        BUG_ON(shift > BITS_PER_LONG);
        if (radix_tree_is_internal_node(entry)) {
            entry_to_node(entry)->parent = node;
        } else if (xa_is_value(entry)) {
            /* Moving a value entry root->xa_head to a node */
            node->nr_values = 1;
        }
        /*
         * entry was already in the radix tree, so we do not need
         * rcu_assign_pointer here
         */
        node->slots[0] = (void __rcu *)entry;
        entry = node_to_entry(node);
        rcu_assign_pointer(root->xa_head, entry);
        shift += RADIX_TREE_MAP_SHIFT;
    } while (shift <= maxshift);
out:
    return maxshift + RADIX_TREE_MAP_SHIFT;
}

/**
 *	radix_tree_shrink    -    shrink radix tree to minimum height
 *	@root		radix tree root
 */
static inline bool radix_tree_shrink(struct radix_tree_root *root)
{
    bool shrunk = false;

    for (;;) {
        struct radix_tree_node *node = rcu_dereference_raw(root->xa_head);
        struct radix_tree_node *child;

        if (!radix_tree_is_internal_node(node))
            break;
        node = entry_to_node(node);

        /*
         * The candidate node has more than one child, or its child
         * is not at the leftmost slot, we cannot shrink.
         */
        if (node->count != 1)
            break;
        child = rcu_dereference_raw(node->slots[0]);
        if (!child)
            break;

        /*
         * For an IDR, we must not shrink entry 0 into the root in
         * case somebody calls idr_replace() with a pointer that
         * appears to be an internal entry
         */
        if (!node->shift && is_idr(root))
            break;

        if (radix_tree_is_internal_node(child))
            entry_to_node(child)->parent = NULL;

        /*
         * We don't need rcu_assign_pointer(), since we are simply
         * moving the node from one part of the tree to another: if it
         * was safe to dereference the old pointer to it
         * (node->slots[0]), it will be safe to dereference the new
         * one (root->xa_head) as far as dependent read barriers go.
         */
        root->xa_head = (void __rcu *)child;
        if (is_idr(root) && !tag_get(node, IDR_FREE, 0))
            root_tag_clear(root, IDR_FREE);

        /*
         * We have a dilemma here. The node's slot[0] must not be
         * NULLed in case there are concurrent lookups expecting to
         * find the item. However if this was a bottom-level node,
         * then it may be subject to the slot pointer being visible
         * to callers dereferencing it. If item corresponding to
         * slot[0] is subsequently deleted, these callers would expect
         * their slot to become empty sooner or later.
         *
         * For example, lockless pagecache will look up a slot, deref
         * the page pointer, and if the page has 0 refcount it means it
         * was concurrently deleted from pagecache so try the deref
         * again. Fortunately there is already a requirement for logic
         * to retry the entire slot lookup -- the indirect pointer
         * problem (replacing direct root node with an indirect pointer
         * also results in a stale slot). So tag the slot as indirect
         * to force callers to retry.
         */
        node->count = 0;
        if (!radix_tree_is_internal_node(child)) {
            node->slots[0] = (void __rcu *)RADIX_TREE_RETRY;
        }

        WARN_ON_ONCE(!list_empty(&node->private_list));
        radix_tree_node_free(node);
        shrunk = true;
    }

    return shrunk;
}

static bool delete_node(struct radix_tree_root *root,
            struct radix_tree_node *node)
{
    bool deleted = false;

    do {
        struct radix_tree_node *parent;

        if (node->count) {
            if (node_to_entry(node) ==
                    rcu_dereference_raw(root->xa_head))
                deleted |= radix_tree_shrink(root);
            return deleted;
        }

        parent = node->parent;
        if (parent) {
            parent->slots[node->offset] = NULL;
            parent->count--;
        } else {
            /*
             * Shouldn't the tags already have all been cleared
             * by the caller?
             */
            if (!is_idr(root))
                root_tag_clear_all(root);
            root->xa_head = NULL;
        }

        WARN_ON_ONCE(!list_empty(&node->private_list));
        radix_tree_node_free(node);
        deleted = true;

        node = parent;
    } while (node);

    return deleted;
}

/**
 *	__radix_tree_create	-	create a slot in a radix tree
 *	@root:		radix tree root
 *	@index:		index key
 *	@nodep:		returns node
 *	@slotp:		returns slot
 *
 *	Create, if necessary, and return the node and slot for an item
 *	at position @index in the radix tree @root.
 *
 *	Until there is more than one item in the tree, no nodes are
 *	allocated and @root->xa_head is used as a direct slot instead of
 *	pointing to a node, in which case *@nodep will be NULL.
 *
 *	Returns -ENOMEM, or 0 for success.
 */
static int __radix_tree_create(struct radix_tree_root *root,
        unsigned long index, struct radix_tree_node **nodep,
        void __rcu ***slotp)
{
    struct radix_tree_node *node = NULL, *child;
    void __rcu **slot = (void __rcu **)&root->xa_head;
    unsigned long maxindex;
    unsigned int shift, offset = 0;
    unsigned long max = index;
    gfp_t gfp = root_gfp_mask(root);

    shift = radix_tree_load_root(root, &child, &maxindex);

    /* Make sure the tree is high enough.  */
    if (max > maxindex) {
        int error = radix_tree_extend(root, gfp, max, shift);
        if (error < 0)
            return error;
        shift = error;
        child = rcu_dereference_raw(root->xa_head);
    }

    while (shift > 0) {
        shift -= RADIX_TREE_MAP_SHIFT;
        if (child == NULL) {
            /* Have to add a child node.  */
            child = radix_tree_node_alloc(gfp, node, root, shift,
                            offset, 0, 0);
            if (!child)
                return -ENOMEM;
            rcu_assign_pointer(*slot, node_to_entry(child));
            if (node)
                node->count++;
        } else if (!radix_tree_is_internal_node(child))
            break;

        /* Go a level down */
        node = entry_to_node(child);
        offset = radix_tree_descend(node, &child, index);
        slot = &node->slots[offset];
    }

    if (nodep)
        *nodep = node;
    if (slotp)
        *slotp = slot;
    return 0;
}

/*
 * Free any nodes below this node.  The tree is presumed to not need
 * shrinking, and any user data in the tree is presumed to not need a
 * destructor called on it.  If we need to add a destructor, we can
 * add that functionality later.  Note that we may not clear tags or
 * slots from the tree as an RCU walker may still have a pointer into
 * this subtree.  We could replace the entries with RADIX_TREE_RETRY,
 * but we'll still have to clear those in rcu_free.
 */
static void radix_tree_free_nodes(struct radix_tree_node *node)
{
    unsigned offset = 0;
    struct radix_tree_node *child = entry_to_node(node);

    for (;;) {
        void *entry = rcu_dereference_raw(child->slots[offset]);
        if (xa_is_node(entry) && child->shift) {
            child = entry_to_node(entry);
            offset = 0;
            continue;
        }
        offset++;
        while (offset == RADIX_TREE_MAP_SIZE) {
            struct radix_tree_node *old = child;
            offset = child->offset + 1;
            child = child->parent;
            WARN_ON_ONCE(!list_empty(&old->private_list));
            radix_tree_node_free(old);
            if (old == entry_to_node(node))
                return;
        }
    }
}

static inline int insert_entries(struct radix_tree_node *node,
        void __rcu **slot, void *item, bool replace)
{
    if (*slot)
        return -EEXIST;
    rcu_assign_pointer(*slot, item);
    if (node) {
        node->count++;
        if (xa_is_value(item))
            node->nr_values++;
    }
    return 1;
}

/**
 *	__radix_tree_insert    -    insert into a radix tree
 *	@root:		radix tree root
 *	@index:		index key
 *	@item:		item to insert
 *
 *	Insert an item into the radix tree at position @index.
 */
int radix_tree_insert(struct radix_tree_root *root, unsigned long index,
            void *item)
{
    struct radix_tree_node *node;
    void __rcu **slot;
    int error;

    BUG_ON(radix_tree_is_internal_node(item));

    error = __radix_tree_create(root, index, &node, &slot);
    if (error)
        return error;

    error = insert_entries(node, slot, item, false);
    if (error < 0)
        return error;

    if (node) {
        unsigned offset = get_slot_offset(node, slot);
        BUG_ON(tag_get(node, 0, offset));
        BUG_ON(tag_get(node, 1, offset));
        BUG_ON(tag_get(node, 2, offset));
    } else {
        BUG_ON(root_tags_get(root));
    }

    return 0;
}

/**
 *	__radix_tree_lookup	-	lookup an item in a radix tree
 *	@root:		radix tree root
 *	@index:		index key
 *	@nodep:		returns node
 *	@slotp:		returns slot
 *
 *	Lookup and return the item at position @index in the radix
 *	tree @root.
 *
 *	Until there is more than one item in the tree, no nodes are
 *	allocated and @root->xa_head is used as a direct slot instead of
 *	pointing to a node, in which case *@nodep will be NULL.
 */
void *__radix_tree_lookup(const struct radix_tree_root *root,
              unsigned long index, struct radix_tree_node **nodep,
              void __rcu ***slotp)
{
    struct radix_tree_node *node, *parent;
    unsigned long maxindex;
    void __rcu **slot;

 restart:
    parent = NULL;
    slot = (void __rcu **)&root->xa_head;
    radix_tree_load_root(root, &node, &maxindex);
    if (index > maxindex)
        return NULL;

    while (radix_tree_is_internal_node(node)) {
        unsigned offset;

        parent = entry_to_node(node);
        offset = radix_tree_descend(parent, &node, index);
        slot = parent->slots + offset;
        if (node == RADIX_TREE_RETRY)
            goto restart;
        if (parent->shift == 0)
            break;
    }

    if (nodep)
        *nodep = parent;
    if (slotp)
        *slotp = slot;
    return node;
}

/**
 *	radix_tree_lookup_slot    -    lookup a slot in a radix tree
 *	@root:		radix tree root
 *	@index:		index key
 *
 *	Returns:  the slot corresponding to the position @index in the
 *	radix tree @root. This is useful for update-if-exists operations.
 *
 *	This function can be called under rcu_read_lock iff the slot is not
 *	modified by radix_tree_replace_slot, otherwise it must be called
 *	exclusive from other writers. Any dereference of the slot must be done
 *	using radix_tree_deref_slot.
 */
void __rcu **radix_tree_lookup_slot(const struct radix_tree_root *root,
                unsigned long index)
{
    void __rcu **slot;

    if (!__radix_tree_lookup(root, index, NULL, &slot))
        return NULL;
    return slot;
}

/**
 *	radix_tree_lookup    -    perform lookup operation on a radix tree
 *	@root:		radix tree root
 *	@index:		index key
 *
 *	Lookup the item at the position @index in the radix tree @root.
 *
 *	This function can be called under rcu_read_lock, however the caller
 *	must manage lifetimes of leaf nodes (eg. RCU may also be used to free
 *	them safely). No RCU barriers are required to access or modify the
 *	returned item, however.
 */
void *radix_tree_lookup(const struct radix_tree_root *root, unsigned long index)
{
    return __radix_tree_lookup(root, index, NULL, NULL);
}

static void replace_slot(void __rcu **slot, void *item,
        struct radix_tree_node *node, int count, int values)
{
    if (node && (count || values)) {
        node->count += count;
        node->nr_values += values;
    }

    rcu_assign_pointer(*slot, item);
}

static bool node_tag_get(const struct radix_tree_root *root,
                const struct radix_tree_node *node,
                unsigned int tag, unsigned int offset)
{
    if (node)
        return tag_get(node, tag, offset);
    return root_tag_get(root, tag);
}

/*
 * IDR users want to be able to store NULL in the tree, so if the slot isn't
 * free, don't adjust the count, even if it's transitioning between NULL and
 * non-NULL.  For the IDA, we mark slots as being IDR_FREE while they still
 * have empty bits, but it only stores NULL in slots when they're being
 * deleted.
 */
static int calculate_count(struct radix_tree_root *root,
                struct radix_tree_node *node, void __rcu **slot,
                void *item, void *old)
{
    if (is_idr(root)) {
        unsigned offset = get_slot_offset(node, slot);
        bool free = node_tag_get(root, node, IDR_FREE, offset);
        if (!free)
            return 0;
        if (!old)
            return 1;
    }
    return !!item - !!old;
}

/**
 * __radix_tree_replace		- replace item in a slot
 * @root:		radix tree root
 * @node:		pointer to tree node
 * @slot:		pointer to slot in @node
 * @item:		new item to store in the slot.
 *
 * For use with __radix_tree_lookup().  Caller must hold tree write locked
 * across slot lookup and replacement.
 */
void __radix_tree_replace(struct radix_tree_root *root,
              struct radix_tree_node *node,
              void __rcu **slot, void *item)
{
    void *old = rcu_dereference_raw(*slot);
    int values = !!xa_is_value(item) - !!xa_is_value(old);
    int count = calculate_count(root, node, slot, item, old);

    /*
     * This function supports replacing value entries and
     * deleting entries, but that needs accounting against the
     * node unless the slot is root->xa_head.
     */
    WARN_ON_ONCE(!node && (slot != (void __rcu **)&root->xa_head) &&
            (count || values));
    replace_slot(slot, item, node, count, values);

    if (!node)
        return;

    delete_node(root, node);
}

/**
 * radix_tree_replace_slot	- replace item in a slot
 * @root:	radix tree root
 * @slot:	pointer to slot
 * @item:	new item to store in the slot.
 *
 * For use with radix_tree_lookup_slot() and
 * radix_tree_gang_lookup_tag_slot().  Caller must hold tree write locked
 * across slot lookup and replacement.
 *
 * NOTE: This cannot be used to switch between non-entries (empty slots),
 * regular entries, and value entries, as that requires accounting
 * inside the radix tree node. When switching from one type of entry or
 * deleting, use __radix_tree_lookup() and __radix_tree_replace() or
 * radix_tree_iter_replace().
 */
void radix_tree_replace_slot(struct radix_tree_root *root,
                 void __rcu **slot, void *item)
{
    __radix_tree_replace(root, NULL, slot, item);
}

/**
 * radix_tree_iter_replace - replace item in a slot
 * @root:	radix tree root
 * @slot:	pointer to slot
 * @item:	new item to store in the slot.
 *
 * For use with radix_tree_for_each_slot().
 * Caller must hold tree write locked.
 */
void radix_tree_iter_replace(struct radix_tree_root *root,
                const struct radix_tree_iter *iter,
                void __rcu **slot, void *item)
{
    __radix_tree_replace(root, iter->node, slot, item);
}

static void node_tag_set(struct radix_tree_root *root,
                struct radix_tree_node *node,
                unsigned int tag, unsigned int offset)
{
    while (node) {
        if (tag_get(node, tag, offset))
            return;
        tag_set(node, tag, offset);
        offset = node->offset;
        node = node->parent;
    }

    if (!root_tag_get(root, tag))
        root_tag_set(root, tag);
}

/**
 *	radix_tree_tag_set - set a tag on a radix tree node
 *	@root:		radix tree root
 *	@index:		index key
 *	@tag:		tag index
 *
 *	Set the search tag (which must be < RADIX_TREE_MAX_TAGS)
 *	corresponding to @index in the radix tree.  From
 *	the root all the way down to the leaf node.
 *
 *	Returns the address of the tagged item.  Setting a tag on a not-present
 *	item is a bug.
 */
void *radix_tree_tag_set(struct radix_tree_root *root,
            unsigned long index, unsigned int tag)
{
    struct radix_tree_node *node, *parent;
    unsigned long maxindex;

    radix_tree_load_root(root, &node, &maxindex);
    BUG_ON(index > maxindex);

    while (radix_tree_is_internal_node(node)) {
        unsigned offset;

        parent = entry_to_node(node);
        offset = radix_tree_descend(parent, &node, index);
        BUG_ON(!node);

        if (!tag_get(parent, tag, offset))
            tag_set(parent, tag, offset);
    }

    /* set the root's tag bit */
    if (!root_tag_get(root, tag))
        root_tag_set(root, tag);

    return node;
}

static void node_tag_clear(struct radix_tree_root *root,
                struct radix_tree_node *node,
                unsigned int tag, unsigned int offset)
{
    while (node) {
        if (!tag_get(node, tag, offset))
            return;
        tag_clear(node, tag, offset);
        if (any_tag_set(node, tag))
            return;

        offset = node->offset;
        node = node->parent;
    }

    /* clear the root's tag bit */
    if (root_tag_get(root, tag))
        root_tag_clear(root, tag);
}

/**
 *	radix_tree_tag_clear - clear a tag on a radix tree node
 *	@root:		radix tree root
 *	@index:		index key
 *	@tag:		tag index
 *
 *	Clear the search tag (which must be < RADIX_TREE_MAX_TAGS)
 *	corresponding to @index in the radix tree.  If this causes
 *	the leaf node to have no tags set then clear the tag in the
 *	next-to-leaf node, etc.
 *
 *	Returns the address of the tagged item on success, else NULL.  ie:
 *	has the same return value and semantics as radix_tree_lookup().
 */
void *radix_tree_tag_clear(struct radix_tree_root *root,
            unsigned long index, unsigned int tag)
{
    struct radix_tree_node *node, *parent;
    unsigned long maxindex;
    int uninitialized_var(offset);

    radix_tree_load_root(root, &node, &maxindex);
    if (index > maxindex)
        return NULL;

    parent = NULL;

    while (radix_tree_is_internal_node(node)) {
        parent = entry_to_node(node);
        offset = radix_tree_descend(parent, &node, index);
    }

    if (node)
        node_tag_clear(root, parent, tag, offset);

    return node;
}

/**
  * radix_tree_iter_tag_clear - clear a tag on the current iterator entry
  * @root: radix tree root
  * @iter: iterator state
  * @tag: tag to clear
  */
void radix_tree_iter_tag_clear(struct radix_tree_root *root,
            const struct radix_tree_iter *iter, unsigned int tag)
{
    node_tag_clear(root, iter->node, tag, iter_offset(iter));
}

/**
 * radix_tree_tag_get - get a tag on a radix tree node
 * @root:		radix tree root
 * @index:		index key
 * @tag:		tag index (< RADIX_TREE_MAX_TAGS)
 *
 * Return values:
 *
 *  0: tag not present or not set
 *  1: tag set
 *
 * Note that the return value of this function may not be relied on, even if
 * the RCU lock is held, unless tag modification and node deletion are excluded
 * from concurrency.
 */
int radix_tree_tag_get(const struct radix_tree_root *root,
            unsigned long index, unsigned int tag)
{
    struct radix_tree_node *node, *parent;
    unsigned long maxindex;

    if (!root_tag_get(root, tag))
        return 0;

    radix_tree_load_root(root, &node, &maxindex);
    if (index > maxindex)
        return 0;

    while (radix_tree_is_internal_node(node)) {
        unsigned offset;

        parent = entry_to_node(node);
        offset = radix_tree_descend(parent, &node, index);

        if (!tag_get(parent, tag, offset))
            return 0;
        if (node == RADIX_TREE_RETRY)
            break;
    }

    return 1;
}

/* Construct iter->tags bit-mask from node->tags[tag] array */
static void set_iter_tags(struct radix_tree_iter *iter,
                struct radix_tree_node *node, unsigned offset,
                unsigned tag)
{
    unsigned tag_long = offset / BITS_PER_LONG;
    unsigned tag_bit  = offset % BITS_PER_LONG;

    if (!node) {
        iter->tags = 1;
        return;
    }

    iter->tags = node->tags[tag][tag_long] >> tag_bit;

    /* This never happens if RADIX_TREE_TAG_LONGS == 1 */
    if (tag_long < RADIX_TREE_TAG_LONGS - 1) {
        /* Pick tags from next element */
        if (tag_bit)
            iter->tags |= node->tags[tag][tag_long + 1] <<
                        (BITS_PER_LONG - tag_bit);
        /* Clip chunk size, here only BITS_PER_LONG tags */
        iter->next_index = __radix_tree_iter_add(iter, BITS_PER_LONG);
    }
}

void __rcu **radix_tree_iter_resume(void __rcu **slot,
                    struct radix_tree_iter *iter)
{
    slot++;
    iter->index = __radix_tree_iter_add(iter, 1);
    iter->next_index = iter->index;
    iter->tags = 0;
    return NULL;
}

/**
 * radix_tree_next_chunk - find next chunk of slots for iteration
 *
 * @root:	radix tree root
 * @iter:	iterator state
 * @flags:	RADIX_TREE_ITER_* flags and tag index
 * Returns:	pointer to chunk first slot, or NULL if iteration is over
 */
void __rcu **radix_tree_next_chunk(const struct radix_tree_root *root,
                 struct radix_tree_iter *iter, unsigned flags)
{
    unsigned tag = flags & RADIX_TREE_ITER_TAG_MASK;
    struct radix_tree_node *node, *child;
    unsigned long index, offset, maxindex;

    if ((flags & RADIX_TREE_ITER_TAGGED) && !root_tag_get(root, tag))
        return NULL;

    /*
     * Catch next_index overflow after ~0UL. iter->index never overflows
     * during iterating; it can be zero only at the beginning.
     * And we cannot overflow iter->next_index in a single step,
     * because RADIX_TREE_MAP_SHIFT < BITS_PER_LONG.
     *
     * This condition also used by radix_tree_next_slot() to stop
     * contiguous iterating, and forbid switching to the next chunk.
     */
    index = iter->next_index;
    if (!index && iter->index)
        return NULL;

 restart:
    radix_tree_load_root(root, &child, &maxindex);
    if (index > maxindex)
        return NULL;
    if (!child)
        return NULL;

    if (!radix_tree_is_internal_node(child)) {
        /* Single-slot tree */
        iter->index = index;
        iter->next_index = maxindex + 1;
        iter->tags = 1;
        iter->node = NULL;
        return (void __rcu **)&root->xa_head;
    }

    do {
        node = entry_to_node(child);
        offset = radix_tree_descend(node, &child, index);

        if ((flags & RADIX_TREE_ITER_TAGGED) ?
                !tag_get(node, tag, offset) : !child) {
            /* Hole detected */
            if (flags & RADIX_TREE_ITER_CONTIG)
                return NULL;

            if (flags & RADIX_TREE_ITER_TAGGED)
                offset = radix_tree_find_next_bit(node, tag,
                        offset + 1);
            else
                while (++offset	< RADIX_TREE_MAP_SIZE) {
                    void *slot = rcu_dereference_raw(
                            node->slots[offset]);
                    if (slot)
                        break;
                }
            index &= ~node_maxindex(node);
            index += offset << node->shift;
            /* Overflow after ~0UL */
            if (!index)
                return NULL;
            if (offset == RADIX_TREE_MAP_SIZE)
                goto restart;
            child = rcu_dereference_raw(node->slots[offset]);
        }

        if (!child)
            goto restart;
        if (child == RADIX_TREE_RETRY)
            break;
    } while (node->shift && radix_tree_is_internal_node(child));

    /* Update the iterator state */
    iter->index = (index &~ node_maxindex(node)) | offset;
    iter->next_index = (index | node_maxindex(node)) + 1;
    iter->node = node;

    if (flags & RADIX_TREE_ITER_TAGGED)
        set_iter_tags(iter, node, offset, tag);

    return node->slots + offset;
}

/**
 *	radix_tree_gang_lookup - perform multiple lookup on a radix tree
 *	@root:		radix tree root
 *	@results:	where the results of the lookup are placed
 *	@first_index:	start the lookup from this key
 *	@max_items:	place up to this many items at *results
 *
 *	Performs an index-ascending scan of the tree for present items.  Places
 *	them at *@results and returns the number of items which were placed at
 *	*@results.
 *
 *	The implementation is naive.
 *
 *	Like radix_tree_lookup, radix_tree_gang_lookup may be called under
 *	rcu_read_lock. In this case, rather than the returned results being
 *	an atomic snapshot of the tree at a single point in time, the
 *	semantics of an RCU protected gang lookup are as though multiple
 *	radix_tree_lookups have been issued in individual locks, and results
 *	stored in 'results'.
 */
unsigned int
radix_tree_gang_lookup(const struct radix_tree_root *root, void **results,
            unsigned long first_index, unsigned int max_items)
{
    struct radix_tree_iter iter;
    void __rcu **slot;
    unsigned int ret = 0;

    if (unlikely(!max_items))
        return 0;

    radix_tree_for_each_slot(slot, root, &iter, first_index) {
        results[ret] = rcu_dereference_raw(*slot);
        if (!results[ret])
            continue;
        if (radix_tree_is_internal_node(results[ret])) {
            slot = radix_tree_iter_retry(&iter);
            continue;
        }
        if (++ret == max_items)
            break;
    }

    return ret;
}

/**
 *	radix_tree_gang_lookup_tag - perform multiple lookup on a radix tree
 *	                             based on a tag
 *	@root:		radix tree root
 *	@results:	where the results of the lookup are placed
 *	@first_index:	start the lookup from this key
 *	@max_items:	place up to this many items at *results
 *	@tag:		the tag index (< RADIX_TREE_MAX_TAGS)
 *
 *	Performs an index-ascending scan of the tree for present items which
 *	have the tag indexed by @tag set.  Places the items at *@results and
 *	returns the number of items which were placed at *@results.
 */
unsigned int
radix_tree_gang_lookup_tag(const struct radix_tree_root *root, void **results,
        unsigned long first_index, unsigned int max_items,
        unsigned int tag)
{
    struct radix_tree_iter iter;
    void __rcu **slot;
    unsigned int ret = 0;

    if (unlikely(!max_items))
        return 0;

    radix_tree_for_each_tagged(slot, root, &iter, first_index, tag) {
        results[ret] = rcu_dereference_raw(*slot);
        if (!results[ret])
            continue;
        if (radix_tree_is_internal_node(results[ret])) {
            slot = radix_tree_iter_retry(&iter);
            continue;
        }
        if (++ret == max_items)
            break;
    }

    return ret;
}

/**
 *	radix_tree_gang_lookup_tag_slot - perform multiple slot lookup on a
 *					  radix tree based on a tag
 *	@root:		radix tree root
 *	@results:	where the results of the lookup are placed
 *	@first_index:	start the lookup from this key
 *	@max_items:	place up to this many items at *results
 *	@tag:		the tag index (< RADIX_TREE_MAX_TAGS)
 *
 *	Performs an index-ascending scan of the tree for present items which
 *	have the tag indexed by @tag set.  Places the slots at *@results and
 *	returns the number of slots which were placed at *@results.
 */
unsigned int
radix_tree_gang_lookup_tag_slot(const struct radix_tree_root *root,
        void __rcu ***results, unsigned long first_index,
        unsigned int max_items, unsigned int tag)
{
    struct radix_tree_iter iter;
    void __rcu **slot;
    unsigned int ret = 0;

    if (unlikely(!max_items))
        return 0;

    radix_tree_for_each_tagged(slot, root, &iter, first_index, tag) {
        results[ret] = slot;
        if (++ret == max_items)
            break;
    }

    return ret;
}

static bool __radix_tree_delete(struct radix_tree_root *root,
                struct radix_tree_node *node, void __rcu **slot)
{
    void *old = rcu_dereference_raw(*slot);
    int values = xa_is_value(old) ? -1 : 0;
    unsigned offset = get_slot_offset(node, slot);
    int tag;

    if (is_idr(root))
        node_tag_set(root, node, IDR_FREE, offset);
    else
        for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++)
            node_tag_clear(root, node, tag, offset);

    replace_slot(slot, NULL, node, -1, values);
    return node && delete_node(root, node);
}

/**
 * radix_tree_iter_delete - delete the entry at this iterator position
 * @root: radix tree root
 * @iter: iterator state
 * @slot: pointer to slot
 *
 * Delete the entry at the position currently pointed to by the iterator.
 * This may result in the current node being freed; if it is, the iterator
 * is advanced so that it will not reference the freed memory.  This
 * function may be called without any locking if there are no other threads
 * which can access this tree.
 */
void radix_tree_iter_delete(struct radix_tree_root *root,
                struct radix_tree_iter *iter, void __rcu **slot)
{
    if (__radix_tree_delete(root, iter->node, slot))
        iter->index = iter->next_index;
}

/**
 * radix_tree_delete_item - delete an item from a radix tree
 * @root: radix tree root
 * @index: index key
 * @item: expected item
 *
 * Remove @item at @index from the radix tree rooted at @root.
 *
 * Return: the deleted entry, or %NULL if it was not present
 * or the entry at the given @index was not @item.
 */
void *radix_tree_delete_item(struct radix_tree_root *root,
                 unsigned long index, void *item)
{
    struct radix_tree_node *node = NULL;
    void __rcu **slot = NULL;
    void *entry;

    entry = __radix_tree_lookup(root, index, &node, &slot);
    if (!slot)
        return NULL;
    if (!entry && (!is_idr(root) || node_tag_get(root, node, IDR_FREE,
                        get_slot_offset(node, slot))))
        return NULL;

    if (item && entry != item)
        return NULL;

    __radix_tree_delete(root, node, slot);

    return entry;
}

/**
 * radix_tree_delete - delete an entry from a radix tree
 * @root: radix tree root
 * @index: index key
 *
 * Remove the entry at @index from the radix tree rooted at @root.
 *
 * Return: The deleted entry, or %NULL if it was not present.
 */
void *radix_tree_delete(struct radix_tree_root *root, unsigned long index)
{
    return radix_tree_delete_item(root, index, NULL);
}

/**
 *	radix_tree_tagged - test whether any items in the tree are tagged
 *	@root:		radix tree root
 *	@tag:		tag to test
 */
int radix_tree_tagged(const struct radix_tree_root *root, unsigned int tag)
{
    return root_tag_get(root, tag);
}

/**
 * idr_preload - preload for idr_alloc()
 * @gfp_mask: allocation mask to use for preloading
 *
 * Preallocate memory to use for the next call to idr_alloc().  This function
 * returns with preemption disabled.  It will be enabled by idr_preload_end().
 */
void idr_preload(gfp_t gfp_mask)
{
    if (__radix_tree_preload(gfp_mask, IDR_PRELOAD_SIZE))
        preempt_disable();
}

void __rcu **idr_get_free(struct radix_tree_root *root,
                  struct radix_tree_iter *iter, gfp_t gfp,
                  unsigned long max)
{
    struct radix_tree_node *node = NULL, *child;
    void __rcu **slot = (void __rcu **)&root->xa_head;
    unsigned long maxindex, start = iter->next_index;
    unsigned int shift, offset = 0;

 grow:
    shift = radix_tree_load_root(root, &child, &maxindex);
    if (!radix_tree_tagged(root, IDR_FREE))
        start = max(start, maxindex + 1);
    if (start > max)
        return ERR_PTR(-ENOSPC);

    if (start > maxindex) {
        int error = radix_tree_extend(root, gfp, start, shift);
        if (error < 0)
            return ERR_PTR(error);
        shift = error;
        child = rcu_dereference_raw(root->xa_head);
    }
    if (start == 0 && shift == 0)
        shift = RADIX_TREE_MAP_SHIFT;

    while (shift) {
        shift -= RADIX_TREE_MAP_SHIFT;
        if (child == NULL) {
            /* Have to add a child node.  */
            child = radix_tree_node_alloc(gfp, node, root, shift,
                            offset, 0, 0);
            if (!child)
                return ERR_PTR(-ENOMEM);
            all_tag_set(child, IDR_FREE);
            rcu_assign_pointer(*slot, node_to_entry(child));
            if (node)
                node->count++;
        } else if (!radix_tree_is_internal_node(child))
            break;

        node = entry_to_node(child);
        offset = radix_tree_descend(node, &child, start);
        if (!tag_get(node, IDR_FREE, offset)) {
            offset = radix_tree_find_next_bit(node, IDR_FREE,
                            offset + 1);
            start = next_index(start, node, offset);
            if (start > max)
                return ERR_PTR(-ENOSPC);
            while (offset == RADIX_TREE_MAP_SIZE) {
                offset = node->offset + 1;
                node = node->parent;
                if (!node)
                    goto grow;
                shift = node->shift;
            }
            child = rcu_dereference_raw(node->slots[offset]);
        }
        slot = &node->slots[offset];
    }

    iter->index = start;
    if (node)
        iter->next_index = 1 + min(max, (start | node_maxindex(node)));
    else
        iter->next_index = 1;
    iter->node = node;
    set_iter_tags(iter, node, offset, IDR_FREE);

    return slot;
}

/**
 * idr_destroy - release all internal memory from an IDR
 * @idr: idr handle
 *
 * After this function is called, the IDR is empty, and may be reused or
 * the data structure containing it may be freed.
 *
 * A typical clean-up sequence for objects stored in an idr tree will use
 * idr_for_each() to free all objects, if necessary, then idr_destroy() to
 * free the memory used to keep track of those objects.
 */
void idr_destroy(struct idr *idr)
{
    struct radix_tree_node *node = rcu_dereference_raw(idr->idr_rt.xa_head);
    if (radix_tree_is_internal_node(node))
        radix_tree_free_nodes(node);
    idr->idr_rt.xa_head = NULL;
    root_tag_set(&idr->idr_rt, IDR_FREE);
}

static void
radix_tree_node_ctor(void *arg)
{
    struct radix_tree_node *node = arg;

    memset(node, 0, sizeof(*node));
    INIT_LIST_HEAD(&node->private_list);
}

static int radix_tree_cpu_dead(int cpu)
{
    struct radix_tree_preload *rtp;
    struct radix_tree_node *node;

    /* Free per-cpu pool of preloaded nodes */
    rtp = &per_cpu(radix_tree_preloads, cpu);
    while (rtp->nr) {
        node = rtp->nodes;
        rtp->nodes = node->parent;
        kmem_cache_free(radix_tree_node_cachep, node);
        rtp->nr--;
    }
    return 0;
}

void __init radix_tree_init(void)
{
    int ret;

    BUILD_BUG_ON(RADIX_TREE_MAX_TAGS + __GFP_BITS_SHIFT > 32);
    BUILD_BUG_ON(XA_CHUNK_SIZE > 255);
    radix_tree_node_cachep = kmem_cache_create("radix_tree_node",
            sizeof(struct radix_tree_node), 0,
            SLAB_PANIC, radix_tree_node_ctor);
    ret = cpuhp_register_callback(radix_tree_cpu_dead, "lib/radix:dead", CPUHP_RADIX_DEAD);
    WARN_ON(ret < 0);
}
